The present invention relates to an improved electronic system for an ultrasonic leak detector, and more particularly such an electronic system for use in connection with a high speed can top leak detection mechanism.
A separate invention comprising an improvement in the mechanical apparatus for ultransonic leak testing is the subject of a separate patent application by G. Mosher. That application Ser. No. 772,330 is entitled "Ultrasonic Leak Hole Detection Apparatus And Method" and was filed Feb. 25, 1977. That application is incoporated by reference herein for its disclosure of the aforesaid mechanical apparatus.
At present, leakage detection systems of various types are used to detect leakage that may occur from containers. In one sensitive leakage detector system a container to be tested is filled with helium gas under pressure and any leaks are detected by a sensitive gas detector instrument such as a gas mass spectrometer. That type of leakage detection, although highly accurate and sensitive, is relatively slow and expensive and may not be suitable for testing containers or container parts, such as can tops, produced at a high rate of production. In another widely used system a fluid, such as compressed air, is pumped into a container and the container submerged in water. An operator looks for bubbles and rejects those containers which show leakage. That system is relatively slow, relatively insensitive to very small leaks, and dependent upon the attentiveness and diligence of the operators.
It has also been suggested that a fluid, such as compressed air, be applied in bursts to one side of a container part and an ultrasonic microphone be located on the other side of the part. The microphone detects the ultrasonic high frequency -- 20,000-100,000 Hz -- sound of the air escaping through any holes in the part being tested. That system is relatively fast and relatively sensitive to small holes. It has been necessary in a noisy factory environment to locate the ultrasonic microphone in a chamber to prevent background factory noise, which may be in the same ultrasonic frequency range, from appearing, to the microphone, as a leaking part.
A major difficulty with that type of ultrasonic leakage detection system is that, although fast compared to helium or bubble detection, it is relatively slow compared to the very high production rates possible in part production. For example, the can ends of "pull tab" cans, consisting of a stamped can end having a pull tab with a ring, may be produced at more than 300 per minute i.e. 5 every second. The prior ultrasonic leakage inspection systems discussed in connection with the prior art mentioned below, have been slower than that rate of production, for example, a maximum speed of inspection of about 100 can tops per minute. The alternatives, using such prior art systems, are (i) to inspect only one can top out of three -- which is not desirable as each can top should be individually inspected for leakage or (ii) have three or more inspection machines for each production machine. The second alternative is also not desirable because of the expense in installation, repair, maintenance and operation of the larger number of inspection machines and the complexity of the conveyor and control systems associated with a plurality of inspection machines.
U.S. Pat. No. 3,399,563 to Helms detects leaks in can ends using exterior pressure against the can end and a microphone within a chamber. The chamber is "hermetically sealed" (col. 4, line 44). Helms suggests that a liquid be applied to the can end to enhance leak noise.
U.S. Pat. No. 3,792,606 to Munger describes a leak detector in which a chamber is employed to reduce background noise. Munger utilizes a screen to enhance leak noise.
U.S. Pat. No. 3,795,137 to Lo describes a method for testing aerosol cans for leaks. The can is formed, filled and sealed and tested for leaks due to internal pressure. The testing device uses a circular table with can spaces created by metal blocks on the peripheral portion. As the table turns, the can enters a tunnel which, at the halfway point, has a microphone recessed in an alcove. The openings provide for can access to the test area while the clearance permits table rotation with minimum friction.
U.S. Pat. No. 3,224,252 to Hamilton is directed to "testing" containers for leaks (col. 1, line 9) and supplies internal pressure to the container to be tested. A pressure equilibrium is established between the container and a "sound box." A container leak will cause gas to flow through the chamber, vibrating a reed, the vibrations being detected by a microphone.
U.S. Pat. No. 3,266,296 to Hall describes three methods for inspecting angular work pieces (such as automobile rims). First, a chamber is created about the exterior of the rim using an inflatable rubber seal. Leak noise is detected by a microphone in the interior of the rim. Secondly, two sealing plates are applied to the inside and outside faces of the rim andpressure is applied through the interior of the rim. Any leaks are detected on the outside by a microphone. Thirdly, a closed chamber is formed through inflatable seals around both the interior and the exterior of a selected portion of the rim and the microphone is within the sealed chamber.
Previously, various signal averaging circuits for averaging an electronic signal to improve the signal to noise ratio have been patented including, for example, U.S. Pat. No. 3,087,487, isssued to M. E. Clynes. These circuits, applied to, for example radar and patient medical testing, allow an improvement in the signal which, in turn, allows an increase in detection sensitivity.